DESCRIPTION

Each of the atomic operations is guaranteed to be atomic in the presence
of interrupts. They can be used to implement reference counts or as
building blocks for more advanced synchronization primitives such as
mutexes.
Types
Each atomic operation operates on a specific type. The type to use is
indicated in the function name. The available types that can be used
are:
int unsigned integer
long unsigned long integer
ptr unsigned integer the size of a pointer
32 unsigned 32-bit integer
64 unsigned 64-bit integer
For example, the function to atomically add two integers is called
atomic_add_int().
Certain architectures also provide operations for types smaller than
“int”.
char unsigned character
short unsigned short integer
8 unsigned 8-bit integer
16 unsigned 16-bit integer
These must not be used in MI code because the instructions to implement
them efficiently may not be available.
MemoryBarriers
Memory barriers are used to guarantee the order of data accesses in two
ways. First, they specify hints to the compiler to not re-order or
optimize the operations. Second, on architectures that do not guarantee
ordered data accesses, special instructions or special variants of
instructions are used to indicate to the processor that data accesses
need to occur in a certain order. As a result, most of the atomic
operations have three variants in order to include optional memory
barriers. The first form just performs the operation without any
explicit barriers. The second form uses a read memory barrier, and the
third variant uses a write memory barrier.
The second variant of each operation includes a read memory barrier.
This barrier ensures that the effects of this operation are completed
before the effects of any later data accesses. As a result, the
operation is said to have acquire semantics as it acquires a pseudo-lock
requiring further operations to wait until it has completed. To denote
this, the suffix “_acq” is inserted into the function name immediately
prior to the “_〈type〉” suffix. For example, to subtract two integers
ensuring that any later writes will happen after the subtraction is
performed, use atomic_subtract_acq_int().
The third variant of each operation includes a write memory barrier.
This ensures that all effects of all previous data accesses are completed
before this operation takes place. As a result, the operation is said to
have release semantics as it releases any pending data accesses to be
completed before its operation is performed. To denote this, the suffix
“_rel” is inserted into the function name immediately prior to the
“_〈type〉” suffix. For example, to add two long integers ensuring that
all previous writes will happen first, use atomic_add_rel_long().
A practical example of using memory barriers is to ensure that data
accesses that are protected by a lock are all performed while the lock is
held. To achieve this, one would use a read barrier when acquiring the
lock to guarantee that the lock is held before any protected operations
are performed. Finally, one would use a write barrier when releasing the
lock to ensure that all of the protected operations are completed before
the lock is released.
MultipleProcessors
The current set of atomic operations do not necessarily guarantee
atomicity across multiple processors. To guarantee atomicity across
processors, not only does the individual operation need to be atomic on
the processor performing the operation, but the result of the operation
needs to be pushed out to stable storage and the caches of all other
processors on the system need to invalidate any cache lines that include
the affected memory region. On the i386 architecture, the cache
coherency model requires that the hardware perform this task, thus the
atomic operations are atomic across multiple processors. On the ia64
architecture, coherency is only guaranteed for pages that are configured
to using a caching policy of either uncached or write back.
Semantics
This section describes the semantics of each operation using a C like
notation.
atomic_add(p, v)
*p += v;
atomic_clear(p, v)
*p &= ~v;
atomic_cmpset(dst, old, new)
if (*dst == old) {
*dst = new;
return 1;
} else
return 0;
The atomic_cmpset() functions are not implemented for the types “char”,
“short”, “8”, and “16”.
atomic_fetchadd(p, v)
tmp = *p;
*p += v;
return tmp;
The atomic_fetchadd() functions are only implemented for the types “int”
and “32” and do not have any variants with memory barriers at this time.
atomic_load(addr)
return (*addr)
The atomic_load() functions always have acquire semantics.
atomic_readandclear(addr)
temp = *addr;
*addr = 0;
return (temp);
The atomic_readandclear() functions are not implemented for the types
“char”, “short”, “ptr”, “8”, and “16” and do not have any variants with
memory barriers at this time.
atomic_set(p, v)
*p |= v;
atomic_subtract(p, v)
*p -= v;
atomic_store(p, v)
*p = v;
The atomic_store() functions always have release semantics.
The type “64” is currently not implemented for any of the atomic
operations on the arm, i386, and powerpc architectures.

RETURNVALUES

The atomic_cmpset() function returns the result of the compare operation.
The atomic_fetchadd(), atomic_load(), and atomic_readandclear() functions
return the value at the specified address.

HISTORY

The atomic_add(), atomic_clear(), atomic_set(), and atomic_subtract()
operations were first introduced in FreeBSD 3.0. This first set only
supported the types “char”, “short”, “int”, and “long”. The
atomic_cmpset(), atomic_load(), atomic_readandclear(), and atomic_store()
operations were added in FreeBSD 5.0. The types “8”, “16”, “32”, “64”,
and “ptr” and all of the acquire and release variants were added in
FreeBSD 5.0 as well. The atomic_fetchadd() operations were added in
FreeBSD 6.0.